Soft group - Intercepting System Errors in Basic and TR-DOS when programming in
assmblere (procedure).
Programmers who try to write
high-quality programs to run on
computers compatible with the ZX Spectrum,
faced with some problems of methodological and practical plan.
One of them, perhaps the most annoying is the fact that
when the any read / write drive system
TR-DOS can clear the screen and the message that there was a
disk error or that the diskette installed security token from
the record. After that, she asked whether to try the operation
again, interrupt it or ignore it. For the programmer, a problem
that appears the need to restore the screen after
Treatment for DOS. In addition, the system changes the number
of the current flow. In general, a program running in, say,
windowed interface should be fully processed his mistakes in
this interface.
Next I'll show you how to intercept functions described
TR-DOS better, in my view method. Unfortunately, in this
If you can not select a certain covering all possible
situations the algorithm therefore be given a concrete example,
taken from the windowing applied
programs developed by the author. But first a few words about
how the TR-DOS ROM is drawn to the main computer.
As you know, shadow ROM system
TR-DOS is connected to the place of the main ROM
computer when accessing the processor to certain memory
addresses to read the operation code. One of these addresses is
the address of # 3D13 - the entry point to serve TR-DOS. This
connection sets up a special trigger in Beta Disk Interface,
and additional ROM is connected to the treatment
CPU for the operation code beyond this
Rom. Then the trigger is reset.
If the TR-DOS why either need to turn to the main memory
(for example, to clear the screen, display characters, etc.),
then she does this as follows:
1) placed on the stack the return address for the current
part or module TR-DOS;
2) placed on the stack address is required routines in the
main ROM;
3) to go at 23,746;
4) because it is the address of RAM
Shadow ROM is disabled (cleared by hardware trigger);
5) at this location is situated instruction RET, therefore,
is taken from the stack address subroutine in the main ROM and
it runs;
6) RET instruction at the end of the running
subroutine will jump back in the shadow ROM system TR-DOS.
The proposed example shows how
You can apply to the TR-DOS, so that no
allow it to clear the screen and give their
Your Message to the bottom window of the screen. Instead of
Your Message of TR-DOS is given in the window
"Communication systems" used window
interface. In the example, there is recourse to
undescribed here windowing procedures. Projecting their routine
similar purpose, you can insert into place calls to these
procedures, their specific calls with similar functions.
In this case, call TR-DOS divided
into 3 phases:
1) preliminary tasks: setting
interceptors;
2) immediately call TR-DOS at # 3D13 (with ability to work
interceptors);
3) completing the operation: the removal of interceptors.
The program is written in assembler mnemonics HiSoft GEN
Assembler Version 4.1. So the first step - installing
interceptors.
DOSPRE SUB A
LD (RSTFLG), A; message still DOS
; Not issued
LD HL, $ $ DOSW
LD (RSTPTR), HL; pointer in the text, and those
windows - to enter characters
CALL CLKON; show cursor
; An hourglass - a long opera, tion "
CALL IM1; transfer system
; In interrupt mode 1
, Followed by steps to install the ne; rehvatchikov
DOSSWP LD HL, DOS_ST; the first area
; Exchange
LD DE, 23746, the second region
LD BC, 3; the length of the
SWAP2 LD A, (DE); exchange content
; Memory areas
LDI
DEC HL
LD (HL), A
INC HL
JP PE, SWAP2
, So the team RET at
; 23746 replaced on the team JP DOSEMT
LD HL, (23633) is a systemic transition; belt CURCHL
LD C, (HL)
INC HL
LD B, (HL); the address formation; botchika display
the current channel
LD DE, (DOS_CH); in DOS_CH address
, Our handler
LD (DOS_CH), BC; rearrange them
LD (HL), D
DEC HL
LD (HL), E
, So the procedure for deriving the current
; Substituted for a special channel. Set; ka interceptor
finished.
RET
; Directly call DOS
; After a single run DOSPRE, you can
; Many times to run DOS; nuzh after all, but run DOSEND
DOS PUSH AF
LD (IY), -1; "no errors"
SUB A; initial value, the return code of TR-DOS
LD (23823), A
LD (23824), A
POP AF
CALL # 3D13; run TR-DOS
LD A, (23823), setting a flag
; CY on return code TR-DOS
AND A
RET Z; if code = 0, then
; CY = 0
SCF
RET
; Area for temporary storage transition team
DOS_ST JP DOSEMT
; Temporary storage registers
DOS_HL DEFW 0
DOS_BC DEFW 0
; Address of the procedure output to a pipe
DOS_CH DEFW DOSRST
; Interceptor access to the main ROM;
And if the module is running, hence, TR-DOS
, Calls the subroutine main ROM
DOSEMT LD (DOS_HL), HL; maintain registration; algebra
LD (DOS_BC), BC
POP BC; is the address of the OS; novnom ROM
PUSH AF
AND A
LD HL, # D6B
SBC HL, BC; address = # D6B?
; (Sub clearing the screen)
JR Z, DOSCLS; if so, run, eat new module cleanup
AND A
LD HL, # 1601, Address = # 1601?
; (Sub-setting the current flow)
SBC HL, BC
JR Z, DOSIGN; if so, ignored; fix the call flow does
not change ; Called routine is not one of
; Intercepted, so it ipolnyaem
POP AF
PUSH BC; vozvrashenie address on
; Place
LD HL, (DOS_HL); return result; Giustra
LD BC, (DOS_BC)
RET; execution ordered
; Sub main ROM
DOSIGN POP AF
RET
; Non-zero value of this flag indicates
; That the message box TR-DOS is already open, then
RSTFLG DEFB 0
; Address space of the next character in the
The text box
RSTPTR DEFW $ $ DOSW
; Routine cleaning windows, replacing
; Routine cleaning of the screen
DOSCLS POP AF; concrete action, not principal conditions
LD HL, $ $ DOSW
LD DE, $ $ DOSW +1
LD BC, 5 * 1.21
LD (HL), 255
LDIR
LD HL, $ $ DOSW
LD (RSTPTR), HL
LD A, (RSTFLG); if the window is not on, indoor
AND A
LD A, 28
CALL Z, OPEN; then open it
LD A, -1
LD (RSTFLG), A; change the flag
JP REDRAW; redraw neighborhood, but
; Subroutine output symbol in the window replacement; sculpt a
character output in the region "K" ; Screen
DOSRST PUSH BC
PUSH DE
PUSH HL
LD HL, (RSTPTR)
LD (HL), A
INC HL
LD (RSTPTR), HL
CALL TEXTON; update the text in the
; Window
POP HL
POP DE
POP BC
RET
; Cleanup operations after a series of calls
; DOS
DOSEND LD (IY), -1; "no error occurred"
CALL IM2; return mode
; Interrupt 2
CALL CLKOFF; remove cursor
; Hourglass
CALL DOSSWP; bring back all
; Vector conversion and Indices
LD A, (RSTFLG); if the window messages; of TR-DOS
revealed
AND A
CALL NZ, CLOSE; then close it
RET
Thus, if the TR-DOS wants to
during operation to give a message Disk
error or Read only, it is the first step
attempt to clean the screen. This attempt will be detected
DOSEMT and instead of cleaning screen will be executed
withdrawal message box TR-DOS and its purification.
Further, TR-DOS tries to open a stream associated with the
channel "K" for issuing the message. This attempt also
will be caught and ignored.
After this transition at # 10
system will print a message.
It would seem possible at the same subprogram
DOSEMT "catch" and this situation and do not capture the output
in the current channel. Experiments have shown that it works,
but for output numbers TR-DOS uses the procedure
OUT_NUM, which accounts in which case
also intercept and emulate that
awkward and inelegant. It is much easier to intercept the
output channel at the level of the leader tape channel, as done
in DOSSWP.
As a result of TR-DOS message appears in a beautiful
multi-color box in the middle of the screen, not the bottom.
The user can respond to a request in the same way as always. In
addition, the screen is not broken, that very convenient for
the programmer. Upon completion of the sequence of calls to
TR-DOS window closes (in the procedure DOSEND).
Consult the author
You may, by writing to:
257010 Ukraine, Cherkassy-10, PO Box 1529
Kuritsyna BA
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Soft group - Intercepting System Errors in Basic and TR-DOS when programming in
assmblere (procedure).
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